The invention relates to devices that grip and maintain ropes in a desired position, and in particular to locking ropes used in suspension systems in theatrical applications, such as for suspending lights, curtains, scenery and other equipment in theaters. In particular, the devices include a novel automatic safety mechanism which prevents runaways due to an out-of-balance system.
In theatrical productions, displays, exhibitions, and other events, rigging systems are used to suspend scenery, curtains, lights, and other equipment. A counterweight rigging common in theatrical applications, is typically comprised of a head block, floor block, counterweight arbor, loft blocks, lift lines, a rope handline, and a rope lock. Previous attempts to control rope movement include rope locks and clutches, the latter only gripping rope in one direction. When the rope is pulled in the opposite direction, the clutch allows it to slip through. The Kuebler system (U.S. Pat. No. 2,991,526) is neither a clutch nor a rope lock, but only a safety mechanism. In the Kuebler system, a “dead man” pedal defeats the safety mechanism by removing the surface a shoe bears against. Another limitation is that the Kuebler system will not work on a 3-strand rope.
When counterweights are being added or removed, or when a load is added or removed from the lift lines, and the system is out of balance, the goal of a rope lock is to prevent uncontrolled movement of the rope, load, and counterweights. It is also desirable for the rope lock to automatically lock the rope in position and hold it, even if the operator opens the handle. However, rope locks suffer from one or more of the following deficiencies. They may lack the capability to lock the rope in place by manually moving a handle, lack the capability to automatically lock the rope in place in response to a sudden imbalance in the system or a runaway situation, and they may use jaws to grip the rope in a maimer that tends to cut into, pinch, and otherwise cause damage to the rope. In other words, some rope locks have no safety system, or have safety systems that need improvement.
A rope lock system is disclosed in FIG. 1 of U.S. Pat. No. 5,531,297 (Pipan). The item to be lifted (the load) is attached to the lift lines on stage, which are run through loft blocks overhead and to a counterweight arbor. Weight is added to the counterweight arbor to roughly balance the weight of the load plus necessary hardware. A rope or handline for raising and lowering the item is connected to the counterweight arbor, run up to and through the head block, down through a rope lock, then through a floor block, and back to the counterweight arbor. In a balanced system, the load can be raised or lowered with very little force applied to the handline. If the system is not balanced, it may require a substantial force on the handline to prevent either the load or weight arbor from crashing onto the floor and other equipment. To prevent this dangerous possibility, a rope lock or safety system is used to prevent the rope from moving as a result of an imbalance. The Pipan rope lock employs jaws that pivot under direct control of a handle. A disadvantage of this system is that under strong tension, the jaws pinch the rope contributing to rope destruction and dangerous accidents. What is needed is an improved rope lock with an automatic safety system.